Resonant Raman scattering in germanium and zincblende-type semiconductors temperature dependence

We have measured the resonance in the Raman scattering near the E₁ gaps of InAs and of a Ge_0.77 Si_0.23 alloy at 77, 300 and 594°K. In contrast to the E₁ gap determined in absorption and transmission measurements, the coresponding peak in the spectral dependence of the scattering cross section shifts very little with temperature; it occurs at all temperatures very near the energy of the absorption peak measured at low temperatures (∼ 77°K).     

Pressure dependence of the Raman scattering in the ordered phase of NH4Cℓ

We have observed Raman scattering in NH₄C? from 105K to 295K and up to 7kbar hydrostatic pressure, including the disordered phase II and the ordered phase IV. The pressure dependences of internal and lattice modes are reported, as well as that of the libration mode. The results are applied to several theories of the potential barrier to rotation.     

Pressure dependence of the Néel temperature in CeMg and CeZn

Magnetization measurements have been carried out under hydrostatic pressures up to 6 kbar and constant magnetic fields (10 kOe) in CsCl-type compounds: CeMg and CeZn. a decrease in the Néel temperature, T_N, with increasing pressure is observed in both compounds (dT_N/dp=- 0.2 and -0.17 K kbar^-1 respectively in CeMg and CeZn). The relative decrease of T_N has a similar order of magnitude to that found in other cerium compounds with Kondo-type properties.     

Raman scattering in dried DNA and crystalline amino acids

Raman spectrum characteristics of dried deoxyribonucleic acid (DNA) and two types of crystalline amino acids (L-lysine, D-asparagine) are compared in a wide range of frequencies, including the regions of lattice (7 to 200 cm^?1) and intramolecular (200 to 4000 cm^?1) vibrations. It is found that the spectral position of the low-frequency band in the Raman spectrum of DNA with a peak near 26 cm^?1 correlates with the Raman spectrum of high-Q low-frequency modes that manifest themselves in the crystalline amino acids under investigation. The low-frequency band of DNA refers to a twist-like vibrational mode of nucleobases. The intensities of this DNA mode and the high-Q lattice modes of the crystalline amino acids L-lysine and D-asparagine are several times as high as those of the Raman lines corresponding to the intramolecular modes. Resonant coupling of low-frequency modes of DNA and amino acid molecular chains is analyzed.     

Raman scattering and optical absorption studies of an orbital ordered Ca2RuO4

We have reported the Raman scattering and infrared absorption results on a t_2g orbital ordered Ca₂RuO₄. At 10 K, a strong and clear peak was observed in Raman scattering near 1360 cm⁻¹ with x′x′ geometry. In contrast to optic phonon modes, the peak does not show any frequency shift but rapidly decreases with increasing temperature. In addition, the peak is not observed in infrared absorption measurement. By comparing the previous Raman scattering results for several transition metal oxides, we have discussed the possible origins and ambiguities of the intriguing peak in Ca₂RuO₄.     

Pressure and temperature dependence of optical phonons in La0.75Ca0.25MnO3

We present the results of extensive studies on Raman and infrared active phonons in the La_0.75Ca_0.25MnO₃ manganite over a wide temperature (100–320 K) and pressure (0–14 GPa) range. The analysis of the temperature dependent data allows to identify a clear spectroscopic signature of the insulator to metal transition. Indeed the abrupt reduction of the effective electron–phonon interaction on entering the metallic phase determines a change in slope in the temperature dependence of the Jahn–Teller phonon line width. The analysis of the pressure dependent data shows that the octahedral Jahn–Teller distortion, and consequently the electron–phonon interaction, is strongly reduced only in the low-pressure regime. At very high pressure, the onset of a pressure-activated localizing mechanism efficiently contrasts the natural delocalizing tendency of pressure. We finally guess that this effect could be attributed to charge-localizing antiferromagnetic interactions activated by the strong lattice compression.     

Second-order Raman scattering and infra-red absorption in SnS2

Summary The second-order Raman scattering and infra-red absorption spectra of 2H−SnS₂ are reported. The temperature, excitation wavelength and polarization dependences of Raman spectra are studied; resonance effects are put in evidence. The assignment of the second-order bands is discussed on the basis of the appliable selection rules. The role of phonons of high symmetry points different from Γ in second-order spectra is also taken into account. Work partially supported by M.P.I.     

Indirect exciton absorption and Raman scattering in GaAs-AlGaAs superlattices

Longitudinal optic (LO) phonon assisted indirect exciton creation (X_LO), hot carrier relaxation ((e-h)_LO) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X_LO, (e-h)_LO and Raman peaks are observed. The X_LO absorption peaks are identified from the observation of a clear threshold in PLE at ℏω_LO (36.4 meV) above the heavy hole exciton peak. The intensity of X_LO is a maximum at 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above ℏω_LO. The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.     

Raman scattering and optical susceptibilities of Nd-doped glasses

The absolute Raman scattering cross-sections of three Neodymium-doped glasses have been measured. These cross-sections have been used to determine the nonlinear optical susceptibility tensor of the three glasses. We have estimated the relative contributions of the “electronic” and “nuclear” nonlinearities to self-focusing index for linearly polarized light in these glasses. We have also performed numerical computations of the Hibert transform of our Raman scattering cross-sections, which are useful in the prediction of any other nonlinear effects. Work supported in part by the National Science Foundation, and in part by the U.S. Atomic Energy Commission under the University of California Lawrence Livermore Laboratory subcontract No. 2713005.     

Quantum interference between Raman scattering and single-photon absorption

A new kind of quantum interference between Raman scattering and single-photon absorption is predicted theoretically, which gives an expanded view of quantum interference. Its potential application is also proposed.     

Temperature dependence of Raman scattering in Co-doped AlN whiskers

Raman studies are reported for the A ₁ (TO), E ₂ (high) and E ₁ (LO) phonons of Co-doped AlN whiskers from 78 K to 778 K. The temperature dependence of the Raman shifts and line widths of these phonons can be well described by an empirical model which takes into account the contribution of the thermal expansion of the lattice and the symmetric decay of phonons into two and three identical phonons with lower energy. Our results show that the three-phonon process is the dominant decay channel in A ₁ (TO), E ₂ (high) and E ₁ (LO). The symmetric decay of A ₁ (TO) may be attributed to the production of two LA phonons near the M symmetry point of the Brillouin zone. For E ₂ (high), the symmetric decay may be near the M and L symmetry points. The E ₁ (LO) phonon, whose decay behavior has not been identified in AlN, may decay near the H and K symmetry points of the Brillouin zone. Compared with undoped AlN, the stronger temperature dependence of the A ₁ (TO) and E ₂ (high) phonons in Co-doped AlN whiskers was observed, which is probably due to the distortion of the lattice and the much larger tensile stress after doping Co into AlN.     

Pressure dependence of the magnetisation and Mn nmr of Mn4N

Abstract

The expanded lattice material Mn₄N contains two types of Mn site. The ⁵⁵Mn NMR at the Mn(I) site has been measured to 9 kbar at 77K. The Mn(I) NMR frequency, and the magnetisation, increased with pressure which is attributed to a decrease in the magnitude of the moment at the Mn(II) site.     

Raman scattering of light and IR absorption in carbon nanotubes

Raman light scattering and IR absorption spectra of samples containing multilayer carbon nanotubes in different stages of purification by the selective oxidation technique have been investigated. It was found that the Raman spectra of carbon nanotubes exhibit softening of the mode at 1582 cm⁻¹ corresponding to E _2g vibrations of graphite hexagons and a line at 120 cm⁻¹ due to the radial vibrations of nanotubes. In IR absorption spectra measured in the region of 0.07–0.3 eV, several sets of lines with a spacing of 15 meV (120 cm⁻¹) between lines of each group have been detected. We suggest that each group corresponds to electron transitions generating electron-hole pairs in semiconducting nanotubes and contains a phononless 00-line and its phonon replicas with spacing between them equal to the “breathing” mode energy of 120 cm⁻¹. Measurements of electric conductivity at a frequency of 9300 MHz indicate that, in addition to semiconducting nanotubes, the samples contain nanotubes with properties of a highly disordered semimetal. Zh. éksp. Teor. Fiz. 113, 1883–1891 (May 1998)